Gel coats are high performance resins, in particular unsaturated polyester (UPE) resins, that are applied to the outside of a product/article for aesthetics but also to provide resistance to weathering. Gel coats may be clear or colored. Colored gel coats typically contain a high percentage of fillers such as talc, calcium carbonate, titanium dioxide or other pigments.
The use of fumed silica in gel coats, especially in unsaturated polyester (UPE) resins, is an old and very well known technology. Precipitated silica has also been utilized from at least the early 1980's. In 1985, the J. M. Huber Corporation was granted a U.S. Pat. No. 4,497,918 for utilizing combinations of precipitated and fumed silica in thixotropic unsaturated polyester formulations. Similar marine gel coats are disclosed in US 200710001343.
Both fumed silica and standard precipitated silica have limitations when used in gel coat formulations for marine or other corrosive environments. Fumed silica tends to cause porosity in the cured gel coat. The presence of porosity can undermine the quality of corrosion resistant resins because porosity makes a coating susceptible to accelerated weathering and blistering when exposed to light, water, solvents, etc.. Standard precipitated silica can vastly reduce the presence of porosity, but can lead to a greater level of osmostic pressure and thus a faster rate of weathering.
Compared to precipitated silica, fumed silica is more efficient at adding viscosity, is more thixotropic, and provides better clarity. On the other hand advantages of precipitated silica compared to fumed silica include faster and shear independent dispersion, lower cost, better leveling of the coating or casting, and in some cases less viscosity drift. As consequence in many cases, a blend of fumed and precipitated silica is utilized to achieve the benefits of both silica types. This, however, is not an acceptable situation for manufacturers of gel coats because two different kinds of silica have to be purchased and stored, which causes the need of two different storing devices. In addition, it is still necessary to use at least partly fumed silica having a much higher price compared to precipitated silica.
Other measures that have been used to prevent blushing, in particular of watercrafts, include applying an initial clear gel coat composition to the watercraft mold surface followed by a pigmented and filled gel coat composition applied behind the clear coat, or applying a pigmented and filled gel coat composition to the watercraft mold surface and coating the finished demolded watercraft part (e.g., a hull) with an exterior automotive paint. Both these measures require extra time and materials, and introduce an additional required interface in the finished watercraft part.
In summary, therefore, it can be stated that there is a strong need for new gel coats suitable to reduce costs and to simplify the production process.